Consequences of Natal Choice by Female Bean Beetles

Objectives

· Design and perform a set of experiments to evaluate whether female bean beetles (Callosobruchus maculatus) discriminate between their natal host and another suitable species of bean.

· Design and conduct an experiment to evaluate the consequences of bean species choice by female bean beetles.

Introduction

Bean beetles (cowpea seed beetles), Callosobruchus maculatus, are agricultural pest insects of Africa and Asia. Females lay their eggs on the surface of beans (Family Fabaceae). Eggs are deposited (=oviposition) singly and several days after oviposition, a beetle larva (maggot) burrows into the bean. Larval growth and pupation occur inside the bean and are consequently difficult to observe. At 30°C, pupation and emergence of an adult beetle occurs 25-30 days after an egg was deposited. Adults are mature 24 - 36 hours after emergence and they do not need to feed. Adults may live for 1-2 weeks during which time mating and oviposition occur. Since larvae cannot move from the bean on which an egg was deposited, the oviposition choice of a female determines the future food resources available to their offspring. As a result, it is the most critical choice a female makes for her offspring, because it will influence their growth, survival, and future reproduction (Mitchell, 1975; Wasserman and Futuyma, 1981). Although females can be induced to lay eggs (oviposit) on a wide range of bean species, very few bean species result in normal development and the successful emergence of adults. Some bean species are very clearly toxic to Callosobruchus maculatus larvae (Janzen 1977).

Materials

In class, you will be provided with live cultures of bean beetles containing adults that have been raised on mung beans (Phaseolus aureus) or adzuki beans (Phaseolus angularis). Supplies of organic mung beans and adzuki beans also will be available. Female beetles are easily identified in the live cultures because they have two dark stripes on the posterior of the abdomen, whereas the posterior abdomen of males is uniformly light in color.

Experimental Design

Since the oviposition choices of females influence the survival and future success of their offspring, females may be very sensitive to the species and condition of the beans on which they are depositing eggs. Prior to the laboratory class, each group should design a set of experiments to address whether female bean beetles discriminate between their natal host and another suitable species of bean and the consequences of those choices. Each group will present their designs to the class and common experimental approaches will be discussed.

After you have read the background information and before the laboratory class meeting:

· Describe at least TWO experimental designs for evaluating whether female bean beetles discriminate between mung and adzuki beans.

· Describe an additional experiment to evaluate the consequences of females laying eggs on mung or adzuki beans.

· Predict the outcomes for each experiment.

· Identify and list the variables you would manipulate in each experiment.

· Identify and list the variables you would keep constant in each experiment.

· List the data you would collect to determine if your predictions were true.

· Describe the statistical analyses that you would carry out to test your predictions.

Come to class prepared to present your experimental designs.

Literature Cited

Brown, L. and J.F. Downhower. 1988. Analyses in Behavioral Ecology: A Manual for Lab and Field. Sinauer Associates Publishers, 194 pages.

Janzen, D.H. 1977. How southern cowpea weevil larvae (Bruchidae Callosobruchus maculatus) die on non-host seeds. Ecology 58:921-927.

Mitchell, R. 1975. The evolution of oviposition tactics in the bean weevil, Callosobruchus maculatus F. Ecology 56:696-702.

Wasserman, S.S. and D.J. Futuyma. 1981. Evolution of host plant utilization in laboratory populations of the southern cowpea weevil, Callosobruchus maculatus Fabrivius (Coleoptera: Bruchidae). Evolution 35:605-617.

This experiment was written by C. Beck and L. Blumer (www.beanbeetles.org).